1. Field of the Invention
The present invention relates to a control system of an alternating current motor. More particularly, the present invention relates to an apparatus and method for eliminating variable offset values within current detecting signals of a device known as an Application-Specific Intelligent Power Module (hereinafter ASIPM) for a control system of a three-phase alternating current (AC) motor.
2. Description of the Prior Art
The ASIPM is a recently developed device used in controlling the operation of an AC motor. Applicants expect the ASIPM to become a widely accepted control device for next generation AC motors. However, difficulties exist in using ASIPMs with certain types of AC motors. The output signals of a current sensor/detector embedded in the ASIPM often contain variable current offset elements. As a result, the ASIPM may not be easily adapted to some types of AC motor systems since the current offset elements may cause performance reductions in motors containing a torque reduction control, ripple generating control, or the like. In these types of AC motors, the offset is called steady-state deviation or vestigial deviation, and represents a deviation from a desired value even after the control system has recovered to a steady state. Ripple represents an alternating element of a DC output when a rectifier converts an alternating current into direct current.
FIG. 1 is a schematic diagram of a conventional apparatus for eliminating variable offset values of current detecting signals. The conventional apparatus has an ASIPM 1 for detecting feedback currents and associated phases of a three-phase AC motor, and for converting the feedback current and associated phase signals into voltage detecting signals which may contain offset elements, that is, current detecting signals. After determining the phases U and W, the phase V may be calculated, since it is known that the current of the three phases sums to zero. The ASIPM 1 outputs the voltage detecting signals to a CPU 4 via output amplifiers 2, 3. The output amplifiers 2, 3 amplify signals associated with the phases U and W of the voltage detecting signals so as to fit within the input range of a central processing unit (hereinafter CPU) 4. An analog/digital converter 4a within CPU 4 converts the amplified signals of phases U and W into digital signals. The CPU 4 contains a number of adders 4b, 4c, and 4d for adding predetermined offset commands to the digital signals of phases U and W.
As shown in FIG. 1, the conventional apparatus typically needs two current sensors corresponding to the voltage detecting signals at outputs CU and CW, for eliminating variable offset values of current detecting signals. A conventional apparatus may, for certain configurations, require three current sensors corresponding to CU, CW and CV. The conventional apparatus also requires amplifiers 2, 3 for amplifying the current detecting signals, and may require a third amplifier, as well. The current detecting signals through the amplifiers 2, 3 are outputted to the analog/digital converter 4a which is typically embedded in the CPU 4, but may alternatively be independent of the CPU 4. The ASIPM 1 typically functions as a current sensor in addition to functioning as a power device for supplying power to the three-phase AC motor. When the ASIPM 1 functions as a power device, its current sensors are not needed.
FIG. 2 is a flow chart of a conventional method for eliminating variable offset values of current detecting signals. The conventional method of FIG. 2 will be explained with reference to the structure of the apparatus shown in FIG. 1.
In step S01, it is determined whether the ASIPM 1 is being driven in response to the operation of a three-phase motor. If the ASIPM 1 is not being driven, the S01 step proceeds to the end of the method. Otherwise the method proceeds to step S02 where the current and associated phases U and W of feedback signals are detected. The method then proceeds to step S03 in which the current and associated phases U and W are converted into voltage detecting signals containing offset elements, or current detecting elements. After generating the voltage detecting signals in step S03, the method proceeds to step S04 where the phases associated with signals U and W are amplified so as to fit within the input range of the CPU 4. The method then proceeds to step S05, where each current detecting signal (i.e., voltage detecting signal which has an offset element) is converted into a digital value by the analog/digital converter 4a. The method proceeds to step S06, where the CPU 4 calculates the phase associated with the current of signal V. The phase of V may be calculated because it is known that the phases of the three signals U, W and V must sum to zero. The method proceeds to step S07 in which is determined whether the steps S02 through S06 have been performed a predetermined number of times. If the steps S02 through S06 have not been performed a predetermined number of times, the method proceeds from step S07 back to step S02. Upon ascertaining, in step S07, that the steps S02 through S06 have been performed a predetermined number of times, the method proceeds to step S08 where the average value of the voltage detecting signals is determined. In step S09, the CPU 4 stores the average value from step S08 into a memory, and then proceeds to step S10. In step S10, the method proceeds to step S11 if it is determined that the ASIPM 1 is presently being driven, otherwise the method ends. In step S11, the offset elements are eliminated from the current data. Thus, the method of a conventional apparatus results in the current offset elements being eliminated by an initializing function of the CPU 4. The data of each current phase may then be used after eliminating the current offset elements as the ASIPM 1 is being driven.
U.S. Pat. No. 5,319,294, filed on May 24, 1993 ("Ohto et al.") discloses an apparatus for automatically adjusting offset correction values for current detectors. The Ohto et al. device can be used with the above-described conventional apparatus and can be adapted so that the offsets of each current detecting signal may be generated in response to temperature changes or the like while the servomotor is operating. In an AC servomotor, a torque ripple is generated when the offset element is contained within a current detecting signal. The Ohto et al. device seemingly eliminates the effects of torque ripple. Since a servo driver contains an encoder as a position detector, the offset elements contained in the current detecting signals can be calculated by use of a dynamic equation model of the position detector and the servomotor. It appears that phase current data can be acquired by repeatedly subtracting the data for each phase associated with current from the appropriate analog/digital converted current data.
A problem exists in the operation of the CPU of the Ohto et al. device which may result in overcompensation due to the computational lag in using the dynamic model of a motor. Another problem exists because the Ohto et al. device can only be adapted to a synchronous AC motor. In addition, the Ohto et al. device suffers the drawback of being susceptible to the errors of the synchronous motor model. In particular, when offset elements change dramatically, the current detecting signals are shifted up and/or down. This may cause the current detecting signals to deviate from the input range (for instance, from 0V to 5V) of an analog/digital converter. When this situation occurs in the Ohto et al. device, the synchronous motor will not generate the maximum torque.
U.S. Pat. No. 5,053,688 filed on Aug. 16, 1990 ("Rees") discloses a feedback circuit for eliminating DC offset in the drive current of an AC motor. The Rees device appears to rely on the addition of complex hardware to pre-eliminate current offset elements which are contained in signals outputted from a current detector. The offset-eliminated signals then seem to be used as input signals of an analog/digital converter. The Rees device, which controls a motor using chopped-pulse rows as signal sources, may be adapted to brushless DC motors. The Rees device uses a chopper which switches on/off for chopping pulses to convert DC signals into AC signals to control the brushless DC motor by means of the chopped-pulse rows. A circuit for reducing current offset contained in current detecting signals is attached at output ports of the output amplifiers, e.g., the outputs of amplifiers 2 and 3 of FIG. 1. Current offset elements contained in signals detected by current sensors are reduced by external hardware including analog switches, a counter, an integral circuit, or the like. Then the offset-eliminated current signals can be used as input signals of an analog/digital converter.
It seems that the Rees device for eliminating current offset can be set to arbitrarily eliminate a particular effect value, but cannot be adapted to offset elements which are dynamically changing due to temperature variations or the like, while the motor is operating. Since the offset elements in the current detecting signals must be properly modeled by a dynamic model for a motor and a generated ripple, a problem arises in the Rees device in which the arithmetic and logic operations of the CPU may result in over-compensation or under-compensation. Thus, the Rees device suffers from the drawback that it may only be operated accurately in special situations due to its dependance on the accuracy of a dynamic model.